1. Field of the Invention
This invention relates to the fabrication of semiconductor devices, and in particular to a method of aligning a photolithographic mask to a crystal plane.
2. Description of the Prior Art
In the fabrication of semiconductor devices, there are many applications where it is important to be able to precisely align a photo-lithographic mask to the crystal planes of a silicon wafer. Various techniques have been proposed for achieving this result.
In one technique the major and/or the minor flat of the silicon wafer is used to perform the crystal plane alignment. This technique limits the precision of the alignment of the crystal plane of the wafer and the repeatability of the alignment from wafer to wafer because the manufacturers of such silicon wafers do not specify the orientation of these major and/or minor flats to better than about xc2x11xc2x0. Moreover, the mechanical alignment to the major and/or minor flat of the wafer results in even more imprecision in the alignment of that wafer and even more non-repeatability in the alignment from wafer-to-wafer. This alignment technique is widely used by most semiconductor device manufacturers.
An alternative technique is to use X-ray equipment to find the crystal plane of the silicon wafer. In this case, the X-ray equipment would need to be integrated into the photoresist exposure equipment so as to allow a precise location of the crystal plane and a simultaneous alignment of the photo-lithographic mask to the crystal plane in order to perform the transfer of the photo-lithographic pattern onto the silicon wafer.
The article xe2x80x9cPrecise mask alignment to the crystallographic orientation of silicon wafers using wet anisotropic etchingxe2x80x9d, M. Vangbo and Y Backlund, J. Micromech, Microeng, 6 (1996), pp. 279-284 describes a technique using fork structures to align the mask.
The use of a technique involving circular structures is described in the article xe2x80x9cPrecise mask alignment design to crystal orientation of (100) silicon wafer using wet anisotropic etchingxe2x80x9d, P.-H. Chen, C.-M. Hsieh, H.-Y. Peng and M.-K. Chyu, Proceedings of SPIE Vol. 4174 (2000), pp. 462-466.
The above techniques are either insufficiently precise or else not suitable for a high-throughput production process.
According to the present invention there is provided a method of aligning a mask to a specific crystal plane in a wafer, comprising the steps of forming a first mask having at least one alignment structure on a wafer surface, said alignment structure being coarsely aligned with said specific crystal plane and having an array of components that are offset relative to each other by known angles defining the degree of precision with which said mask can be finely aligned with said crystal plane; performing an anisotropic etch through said first mask to etch said alignment structure into said wafer surface, said components of said alignment structure producing different etch patterns in said wafer surface according to their relative orientation to said specific crystal plane; forming a second mask on said wafer surface having a reference structure thereon; and aligning said reference structure on said second mask relative to a said etch pattern identified as being finely aligned with said specific crystal plane.
In accordance with the principles of the invention, a first mask is used to determined the orientation of the desired crystal plane. A second mask can then be used to form index marks in the wafer by aligning the second mask with the etch patterns formed by the first mask that are finely aligned with the desired crystal plane. These index marks can then be used to align subsequent masks used to fabricate functional components in the wafer. If desired, it is possible to form functional patterns on the second mask in addition to the alignment marks.
The above method lends itself to automatic pattern recognition, which is the preferred technique for aligning the second mask with the etch patterns produced by the first mask.
Typically, the aligned patterns are those with symmetrical structures due to the effects of the anisotropic etch on the different crystal planes.
The first mask may contain different types of alignment structure to improve accuracy. In a preferred embodiment, the first mask contains triplet, fork and square structures, which are aligned relative to each other. Other structures that allow quick and easy observation of line shape deformation and asymmetry or that allow independent mask alignment onto patterns with little shape deformation and asymmetry from a wet etch from a wet etch can be employed.
Typically, in the case of silicon a wet etch chemistry that can etch silicon wafer at high rate at particular crystal plane orientations and at lower rate at other particular crystal plane orientations is employed.
The second photo-lithography mask may also contain other structures that allow alignment onto the patterns of the first photo-lithography mask that resulted in little shape deformation and asymmetry from a wet etch.
A pattern recognition device can integrated to apparatus capable of pattern transfer onto the silicon wafer. This can allow an automatic alignment of the second photo-lithography mask onto the structures of the first photo-lithography mask that resulted in little shape deformation and asymmetry from a wet etch;
The present invention makes it possible to achieve a low cost and high throughput alignment between the photo-lithographic masks and the crystal planes of the silicon wafer better than xc2x10.1xc2x0.